The present invention relates generally to halftone images and particularly but not exclusively to a method and apparatus for generating and printing halftone images using ink jet devices susceptible to bi-directional misalignment errors.
A halftone image consists of a selectively positioned arrangement of dots of fixed tone levels, which creates the illusion of a continuous tone, or xe2x80x9ccontonexe2x80x9d, image. Thus, halftoning is a process that allows a continuous tone image to be modified in order that it may be represented by a device that can only represent a finite number of tone levels.
Various halftoning methods have been devised. Initially, xe2x80x9cditheringxe2x80x9d or matrix-based techniques were used to halftone images. In such techniques, a two-dimensional array of values is mapped over a continuous tone image. The data in each pixel in the continuous tone image is compared to the matrix value for that pixel. If the continuous tone value is greater than the matrix value, then a dot is printed in the corresponding position in the halftone image.
Although such halftoning methods are fast, they produce images that are generally considered to be xe2x80x9cgrainyxe2x80x9d. Consequently, the methods used to halftone images have been refined over the years to yield images that are more appealing to the eye.
One such refinement is the so-called xe2x80x9cerror diffusionxe2x80x9d halftoning process. Error diffusion halftoning is a neighborhood process. That is to say, when determining whether or not to print a pixel in a halftone image, the value of the corresponding pixel in the continuous tone image is considered together with information pertaining to pixels neighboring that pixel. This process gives rise to halftone images with reduced grain compared to images produced by dithering or matrix-based techniques. However, in this error diffusion process, the different color planes are halftoned independently. As a consequence, drops in different colors planes may inadvertently coincide, giving rise to dots of a third, undesired color. For example, cyan drops that coincide with magenta drops give rise to dark blue dots.
This effect creates a degree of graininess, which is undesirable. Therefore, the basic error diffusion technique has been further refined to introduce a dependency between the color planes to overcome this problem. xe2x80x9cPlane dependentxe2x80x9d error diffusion utilises an algorithm that ensures that dots in different color planes do not coincide. Thus, through the precise placement of drops, plane dependent error diffusion processes generally produce halftone images that are less grainy and more pleasing to the eye than those produced by the basic error diffusion process.
However, drop placement errors in a printing device may cause a significant decrease in the quality of halftone images produced using a plane dependent error diffusion process. Errors in drop placement may be the result of a range of causes, such as: carriage vibration; paper advance errors; printheads misdirecting drops; paper deformations; and, bi-directional misalignment. Such errors disrupt the precise placement of drops that the plane dependent error diffusion process aims to generate, giving rise to noticeable xe2x80x9cpatterningxe2x80x9d or xe2x80x9clumpinessxe2x80x9d. The decrease in image quality is particularly noticeable if the drop placement errors are caused by a systematic printer error, such as a print carriage vibration or a paper advance error, which tend to produce a very noticeable pattern in the halftone image.
As is well understood in the art, it is often desirable to use bi-directional print modes when printing with ink jet printers in order to increase throughput. By bi-directional print modes, it is meant that swaths are printed as the printheads travel over the print media in each direction, as opposed to unidirectional print modes in which swaths are printed as the printheads travel over the print media only in one direction. However, bi-directional print modes not only tend to exacerbate existing drop placement errors but to cause a further drop placement error problem.
Generally, when an ink drop is ejected from a nozzle of a printhead, it is made up of a two or three individual drops. The first and the largest being generally referred to as the xe2x80x9cheadxe2x80x9d and the remaining one or more drops, which are significantly smaller, being generally referred to as the xe2x80x9ctailxe2x80x9d, or xe2x80x9csatellitexe2x80x9d drops. The drops making up the tail generally travel slower than the head. Consequently, the tail of a drop tends to be printed at a different location on the print media to that of the head. This location is further along the scan axis in the direction of movement of the print carriage at the time that the drop was ejected. When printing in a bi-directional print mode, the effect of satellite drops is to cause an apparent misalignment between drops printed in either direction, even if the heads of drops printed in opposite directions are correctly positioned with respect to each other.
Satellite drops are particularly noticeable when the overall drop volume is small, since at small drop volumes the relative proportion of the tail increases. Currently, the use of drops of ever decreasing drop volumes is increasing, in order to achiever higher print resolution. Therefore, when halftone images are generated using plane dependent error diffusion techniques and printed with bi-directional print modes, the problem of patterning increases.
It would therefore be desirable to provide method and apparatus for producing halftone images, which overcomes one or more of the disadvantages associated with the prior art.
According to a first aspect of the present invention there is provided a method of generating with an inkjet printer system a halftone image from image data comprising a plurality of contone data values, said system comprising a printhead arranged to print drops on a print medium whilst moving relative to said print medium in first and second directions, said method comprising the steps of: generating a first plurality of halftone data, each of said first halftone data being generated from a first proportion of a corresponding contone data value; generating a second plurality of halftone data, each of said second halftone data being generated from a second proportion of a corresponding contone data value; printing drops corresponding to said first plurality of halftone data in said first direction; and, printing drops corresponding to said second plurality of halftone data in said second direction.
In the present invention, the positions of dots printed in each direction of a bi-directional printing process are derived from separate sets of halftone data. Since the separate sets of halftone data are generated in separate processes, the level of dependency between the positioning of drops printed in the two directions may be significantly reduced; i.e. process of determining the positions of one set of dots may be decoupled from that determining the positions of one set of dots.
Thus the present invention provides for a half-toning method and system that is more robust to misalignments. That is to say that where misalignment occurs, the xe2x80x9cpatterningxe2x80x9d and/or apparent change of color resulting from the systematic shift in position of the drops printed in one print direction relative to those printed in the other is less noticeable to the human eye.
The print quality of images generated using the method of the present invention may have slightly increased grain relative to certain prior art methods in which there is dependency between the placement of ink drop printed in each direction and in which no drop placement error is experienced. However, the expense and difficulty of producing printing systems that are free from such errors is in practice often not practicable. Therefore, the present invention is particularly suited for use with printer devices which use pens which are either difficult to align, or devices which are not perfectly aligned. In the first case, printers with a high number of printheads are generally considered difficult to align. In the second case, low cost printers generally have a greater tolerance in pen alignment terms in order to save costs. The present invention is also particularly suited for use in printing images with a high proportion of midtones, which are usually most susceptible to patterning brought about by the combination of systematic drop placement errors and dependent printing.
The present invention also extends to the apparatus corresponding to the claimed methods. Furthermore, the present invention also extends to a computer program, arranged to implement the methods of the present invention.
Other aspects and advantages of the present invention will be apparent from the following detailed description, together with the accompanying drawings.